The present invention is directed to a specific reluctance motor referred to herein as a sinusoidal synchronous reluctance motor (SynRM). A sinusoidal synchronous reluctance motor is a synchronous machine with a well-defined salient pole rotor and a multi-pole stator excited by phased, sinusoidal currents. While such machines have been known for decades, recent advances in the manufacturing techniques for salient pole rotors and advanced current control techniques have only now made the power factor and efficiency of the machine competitive and consequently more attractive for a variety of applications.
Recent renewed interest in synchronous reluctance motors is mainly due to modern field oriented control strategies recently applied to these motors. In particular, it has been shown that a properly designed and field oriented SynRM can perform as well as an induction motor drive when the field weakening range is not too wide. On the other hand, the inherent characteristics of the SynRM make it preferable for some applications. For example, the stator of the SynRM is constructed from a cylindrical structure identical to an induction motor. Hence, the stator of both machines can be constructed from the same assembly line. Further, no starting cage is necessary with an inverter supply. The rotor can therefore be designed purely for synchronous performance.
The electronic control makes a SynRM auto-synchronous and can assure an optimum torque angle at all loads and torques, consequently giving the motor a very high pull-out torque. Also, no damping winding is necessary. This makes it possible to design the motor for the highest reluctance difference X.sub.d -X.sub.q, thereby increasing the power density of the machine. Finally, torque pulsations and acoustic problems are not as severe as those for variable reluctance machines and vector control techniques can be applied in order to achieve high performance.
However, like other ac machines, field orientation control of SynRM requires rotor position information. Particularly, high performance machines calling for torque control need such rotor position information.
A discrete rotor position sensor reduces the reliability and ruggedness of the drive and increases its cost. But, the SynRM possesses unique features which make position sensing much simpler and reliable than either conventional squirrel case induction machines or variable reluctance machines. In contrast to induction machines, the SynRM possesses saliency which permits the rotor position to be sensed since the inductance per phase is a function of rotor position. This allows sensing position at zero speed which is impossible for an induction machine. Secondly, in contrast to the variable reluctance motor, the stator windings of the SynRM are magnetically coupled. Hence, voltages are induced in the stator winding upon open circuit of a phase, which allows for sensing of the coupled voltage. These two features in combination make the task of sensing position easier than for either an induction or variable reluctance motor.
Thus, there remains a need for an indirect rotor position sensing technique and apparatus for a SynRM, utilizing only the input variables (voltages and phase currents) and the rotor saliency information. Further, there remains a need for a simple yet robust rotor position sensing technique that provides for sensing the rest position of the rotor (i.e., at no rotation) during startup operation. Such a rotor position sensing technique and apparatus should require no discreet, specially dedicated components, but rather should use available parameters, such as voltage and phase currents.